The following background discusses three general areas of technology, including thin films interference coatings, thin film filters, and the thermo-optic characteristics of semiconductors and their use in photonic devices.
Thin Film Interference Coatings
Thin film interference coatings (TFIC) represent one of the most mature and widely applied aspects of optical technology. In general, TFIC depends on the deposition of a sequence of one or several (up to hundreds) of thin films, generally transparent over the wavelengths of intended use, with varying refractive indices and other properties in order to obtain desired properties of spectral reflectance and transmittivity, phase shift, or polarization over a given spectral band. For example, anti-reflection coatings have been applied to lenses for almost a century. Other applications of TFIC include narrow bandpass filters, polarizers, color filters, and many others. It is known to the art that a very wide range of optical characteristics can be designed into TFIC, given a sufficient array of starting materials with different indices. Numerous computer simulation and designs tools exist, for example ThinFilm Calc by Spectral Sciences. Widely used deposition processes for TFIC include physical vapor deposition methods such as sputtering or e-beam evaporation. While TFIC are used throughout the field of optics, modem application of TFIC to demanding requirements such as those of the wavelength-division multiplexed (WDM) fiber optic communications industry has become very sophisticated. Using multiple-cavity (up to seven or more) resonantor designs and hundreds of layers, filters with very precise flat-top, steep-sided characteristics are now available that enable small WDM channel spacings (50 GHz or 25 GHz). Other such filters are designed not for their transmission filtering properties but for their spectral distribution of phase delay characteristics over a range of wavelengths, offering precise pulse dispersion or group delay characteristics relevant to high bit-rate networks. TFIC which function as narrowband filters of various kinds will be denoted thin film interference filters, TFIF.
Modern surveys of the field of TFIC in general and TFIF specifically can be found in the following references, as well as numerous journals.    A. Thelen, Design of Optical Interference Coatings, McGraw-Hill, 1989.    J. D. Rancourt, Optical Thin Film Users' Handbook, Macmillan, 1996.    H. A. MacLeod, Thin Film Optical Filters, Second Edn. Macmillan, 1986.    J. A. Dobrowolski, Coatings and Filters, Sect. 8, Handbook of Optics, Second Edn. McGraw-Hill, 1995.    Proceedings of the 2001 OSA Topical Conference on Optical Interference Coatings, July, 2001, Banff, Optical Soc. America.
Since the properties of TFIC are strongly dependent on the refractive indices of the component films, it would be highly desirable to develop “active” thin film materials with controllable or tunable index for TFIC. However, the requirements for such materials are manifold and stringent. To be useful candidates for active thin films, the material would have to offer very low absorption loss and low scattering at the wavelength of interest (as one example, the fiber optic network communications wavelength band near 1.5 μm), be practical for direct thin film deposition in sequential combination with other, passive films of contrasting refractive index through some compatible deposition process, and offer a direct or indirect electrical mechanism of index change that can be effected within a simple and manufacturable physical structure. To be useful, the range of absolute index change must be on the order of a few per cent; it is known that within TFIC designs, the TFIF designs tend to be “resonant” in that they involve Fabry-Perot type single or multiple cavity structures which thereby leverage relatively small index changes in individual layers, on the order of 1% or so, into much larger percentage changes in the net optical characteristics (such as transmission of light) at a given wavelength.
Nevertheless, identifying thin film materials with appropriate characteristics has proven elusive and heretofore there has been no successful technology of tunable TFIC. Obtaining a sufficiently large index modulation in materials with good optical quality is a long standing difficulty in thin film science. The relatively small number of known, practical index control materials may be classified in two groups. High speed materials with small index modulations (on the order of Δn/n=10−5) include electro-optic, piezo-electric, or crystalline semiconductors using charge injection. Most of the attempts at tunable thin film filters to date have been based on such materials. Larger but slower index modulations (Δn/n=10−2) can be achieved with liquid crystals or thermo-optic effects. As recently as July, 2001, Parmentier, Lemarchand, et al, Towards Tunable Optical Filters, Paper WB1, Technical Digest, OSA Topical Mtg. Optical Interference Coatings, Jul. 15–20, 2001, Banff, Alberta, Canada, in a review of possible tunable-index materials for thin film interference coatings compared electro-optic films, piezo-electric films, and oxide thermo-optic films, found no suitable solution. These authors mention but specifically reject the possibility of thermo-optic effects, citing the dielectric films typically used in TFIC, such as tantalum pentoxide and silicon dioxide, whose thermo-optic coefficients are relatively small.
Tunable Filters
Tunable narrowband filters are a commercially important subset of the technology discussed above. Hence, there has been a great deal of research in the field of such filters. A typical requirement in communications is for a filter to tune over the so-called C band, 1528–1561 nm, with a −3dB width on the order of 10 GHz or 0.08 nm and a low insertion loss.
The growth of WDM fiber optic networks has increased demand for a variety of wavelength tunable optical components for diverse network management functions ranging from sources and receivers to dynamic gain equalizers, and dispersion compensators. Tunable optical filters are needed to play several distinct network roles, each with distinct performance requirements. For example, tunable add/drop filters, for which the filter is in the network path, must possess very low insertion loss and ‘flat top’ passband shape. For optical channel monitoring on the other hand, where the filter acts on light tapped off from the network, passband shape and insertion loss are less important than rapid tuning, low cost, compact device footprint and packaging compatible with integration into system modules such as optical amplifiers. These diverse requirements are not met by any one filter technology. Even two filters with identical optical and electrical characteristics may find very different applications if their physical size and shape and manufacturing costs are very different.
Many different approaches to tunable filters have been described, and as is often the case in optical technology, diverse operational principles have been put forward. Tunable filters with comparable passband or tuning range spanning a large range in physical size, form factor, power consumption, complexity and cost are known.
One major category of tunable filters comprise fiber or waveguide based devices. A second category, expanded beam or vertical cavity format tunable filters, are especially desirable and even necessary for certain purposes, in particular when the filter is intended to be integrated with other components in a module, or must be very compact. As indicated in Table I, the micro-electro-mechanical (MEMS) Fabry-Perot is the most widely developed technology in this category, with half a dozen commercial sources.
TABLE IExpanded beam tunable filter technologies. Performance data frompublicly available sourcesLinewidth,insertionTuningMechanismlossrangeLimitsSourcesMEMS0.4 nm45 nmNo multi-cavityCoretek, Axsun,2.5 dBSolus, othersMacro0.2 nm220 nmLarge footprintStocker-YaleInterferometer  3 dB(Optune)Liquid crystal2.5 nm32 nmInsertion loss?Scientific SolnsMechanical0.6 nm35 nmPassband shapeChameleonrotating thin  2 dBSantecfilm filter
Less common approaches to expanded beam filters include liquid crystal devices and mechanically scanned gratings or interferometers. As a group, MEMS Fabry-Perot devices tend to possess wide tuning range, but have an important limitation; they are structurally restricted to the simplest type of single-cavity etalon (Lorentzian passband) design. This means it is impossible to fabricate MEMS filters with more sophisticated designs providing steep skirts for improved adjacent channel rejection, or specific group delay dispersion, or other requirements. Thus they are primarily useful for optical monitoring or tunable-laser applications, but less so for in-path network functions such as add/drop multiplexing, which requires more complex, flat-top, narrow skirt passbands only achievable with multiple cavity resonators.
In this survey it is striking that the most widely used static WDM filter technology, the thin film interference filter TFIF, has not had a practical tunable counterpart except for limited application of mechanically rotated filters. As noted, sophisticated fixed passband TFIF designs incorporating multiple cavities are well known in thin film technology, and the addition of tunability to the many design options of thin film coatings would be highly desirable.
Thermo-optic Use of Semiconductors
It is known that one method of altering the index of optical materials is by varying their temperature. The thermo-optic principle is of interest because, while present in all optical materials to some degree, relatively large effects, on the order of 1% or more, can sometimes be found in materials with very low optical loss in the optical communications bands 1300–1700 nm.
Table II compares the thermo-optic properties of some optoelectronic material families relevant to use in the near infra-red spectrum.
TABLE IIThermo-optic materialsPolymersAcrylates, polyimides (n−1.5) −4 × 10−4/KThin film dielectricsSiO2 (n = 1.44)9.9 × 10−6/KTa2O5 (n = 2.05)9.5 × 10−6/KCrystalline semiconductorsc-GaAs2.5 × 10−4/Kc-Si (n = 3.48, Ghosh, after H. Li)1.8 × 10−4/Kc-Ge (n = 4.11, Ghosh, after H. Li)5.1 × 10−4/KThin film semiconductorsα-Si:H (n = 3.4, Della Corte, sputt.)2.3 × 10−4/Kα-Si:H (n = 3.6, Aegis, PECVD)3.6 × 10−4/K
Thermo-optic polymers including acrylates or polyimides have large (negative) thermo-optic coefficients but can typically only be applied in waveguide forms, as they are not suitable for the deposition processes used for multilayer TFIF. Crystalline semiconductor wafers possess relatively large coefficients, but of course cannot be considered thin films, which for the purpose of this writing means thicknesses of zero to 5 micrometers. With special etching or polishing techniques, wafers can be prepared as thin as 25–50 micrometers, but this process is expensive and difficult to control and handle. In general, crystalline materials grown as wafers are much more difficult to determine as to exact thicknesses than direct-deposited amorphous thin films or epitaxial crystalline thin films, and cannot easily be combined into complex multiple film stacks. Therefore, complex transverse filter structures, for example those with multiple cavity layers cannot be built. Cocorullo and others have demonstrated guided wave components using the thermo-optic properties of thin silicon crystal wafers:    Cocorullo et al, Amorphous Silicon-Based Guided Wave Passive and Active Devices for Silicon Integrated Optoelectronics, IEEE J. Selected Topics Q. E., v.4, p. 997, November/December 1998.    Cocorullo, Della Corte, Rendina, Rubino, Terzini, Thermo-Optic Modulation at 1.5 micron in an α-SiC α-Si α-SiC Planar Guided Wave Structure, IEEE Phot. Tech. Ltrs. 8, p.900, 1996.    Cocorullo, Iodice, et al, Silicon Thermo-Optical Micromodulator with 700 kHZ -3 dB Bandwidth, IEEE Phot. Tech. Ltrs. 7, p.363, 1995    Della Corte, et al, Study of the thermo-optic effect in α-Si:H and α-SiC:H at 1,55 micron, Appl. Phys. Lett., 79, p.168, 2001    Cocorullo et al, Fast infrared light modulation in α-Si:H micro-devices for fiber to the home, J. Non-Crys. Solids, 266, p.1247, 2000.
And other workers have described expanded beam filters based on silicon wafers:    Niemi, Uusimaa, et al., Tunable Silicon Etalon for Simultaneous Spectral Filtering and Wavelength Monitoring of WDM, IEEE Phot. Tech. Ltrs. 13, p. 58, 2001.    Iodice, Cocorullo et al., Simple and Low Cost Technique for Wavelength Division Multiplexing Channel Monitoring, Opt. Eng. 39, p. 1704, 2000
There are no reports of the systematic use of thin film semiconductors, whether amorphous or epitaxial, with use of their thermo-optic properties, as a basis for complex multilayer TFIC or TFIF. In fact, previous writings on TFIF teach away from this very practice because TFIF technologists have avoided temperature sensitive materials in order to create coatings independent of environmental sensitivities. Thus, in the past, the thin film coating industry in general and the WDM TFIF industry in particular has avoided semiconductor materials of any kind in filters primarily because their thermo-optic properties are large and hence coatings made from them would be subject to strong alteration by temperature.